Is it ok to use nitro fuel in a nitro RC car since it has a lower nitro content than what a nitro RC is used to running? Will the different nitro fuel percentage damage the engine?
Whether or not a change in fuel will hurt your RC depends on the recommended fuel for your specific RC, the brand, and the amount of percentage difference. Nitro fuel has three main components: methanol, nitromethane, and oil along with additives such as anti-corrosion agents or degumming, agents added by the various fuel manufacturers. The amount of nitromethane in the fuel is typically about 20 percent but could be anywhere in the 10 to 40 percent range or higher.
The oil that is mixed with a nitro fuel is what helps lubricate all the internal moving parts and keeps the engine running cool. If a nitro engine gets too hot, not only will you see a decline in performance, but worse things can happen. You could permanently damage or ruin the engine. Nitro fuel normally has both castor and synthetic oils in a predefined mixture set by the company that makes it. The percentage of both that are mixed with the nitro fuel is not typically disclosed (although it might be--read the container label). The percentage of oil might range from 8 to 25 percent; 15 to 20 percent is the typical amount of oil found in nitro fuel.
When choosing nitro fuel percentages, first look in the manual that came with your nitro RC and check to see what percentage is recommended. If the manual doesn't list what percentage of nitro to use, ask your local hobby shop for advice as most of the employees have either worked on or own RCs. There are many different brands of nitro RC fuels out there and there is no consensus as to which one works best -- it's trial and error to determine which one works best for your nitro RC.
Keep in mind that the Nitro RC engine size does matter when deciding what percentage of nitro to go with.
Engines that are .21 - .26 or .27 generally use higher percentage nitro fuel in the ranges of 20 to 40 percent.
Is it ok to switch nitro fuels? The best answer: maybe.
I have switched fuels in a pinch when I've run out of what I normally use as long as it was a small change -- 5 percent or so. Going from 10 up to 20 percent can be harmful if your nitro engine is not tuned right for it (leaning out the air/fuel mixture). Going from 20 percent down to 10 will reduce performance and you will probably need to do some additional tuning (richening the air/fuel mixture). But in general, small changes won't usually damage the engine if you pay close attention to how your RC is running and tune the engine as necessary. Avoid making sudden switches to much higher or lower percentage fuels than you normally use and you don't want to constantly switch back and forth.
Ideally, you should stick with the same brand of nitro fuel, even if the nitromethane percentage is different. Each brand may use different types or percentages of oil and other additives so you shouldn't switch brands and nitromethane percentages at the same time.
The bottom line is that switching nitro fuel is a matter of trial and error. It's best not to do so if you are new to nitro RCs. Changing fuels will almost always require re-tuning your engine.